Call Python signal handler

[jdemeyer]

Currently, we are hardcoding what happens when cysignals catches a signal. For SIGINT for example, a KeyboardInterrupt is raised. Instead, we could install a Python-level signal handler and then call that.

We could then also monkey-patch signal.signal to set only the Python-level handler and not the OS handler. Then users could use signal.signal as usual and everything would magically work (despite cysignals handling the signal).